Signal rectifier circuits



Patented Jan. 7, 1941 UNITED STATES PATENT OFFICE SIGNAL RECTIFIER CIRCUITS Application January 15, 1938, Serial No. 185,102

2 Claims.

Our present invention relates to signal rectifier circuits, and more particularly to delayed signal rectification networks adapted for use in automatic volume control (AVC) circuits.

It is known in the automatic volume control art to produce a rectified voltage from a rectifier upon which a signal carrier voltage is impressed, and to use the rectified voltage for decreasing the amplification of one, or more, signal transmission tube stages in the receiver so as to tend to keep the input to the detector more nearly constant in spite of varying signal strength at the antenna, In the simplest form of AVG of this type, the control bias is produced in proportion to the input to the rectifier so that even for signals which are too weak to beamplified up to a desired level at the detector, the automatic volume control action takes place to a certain extent so that weak signals are not brought up to as high a level as would be the case in the absence of AVG. To overcome this drawback it has been proposed in the past to impress a biasing voltage upon the rectifier which produces the AVG voltage so that until the signals impressed upon the detector are brought up to a desired level, there will be no AVC voltage developed. This arrangement, while it takes care of the aforementioned difiiculty, introduces a new drawback: for strong signals the AVG voltage developed remains less than the peak value of the alternating voltage applied to the AVG rectifier by an amount equal to the amount of biasing or delay, voltage introduced. Thus, in such a system the very strong signals are not held down to so constant a level at the detector as they are in the case Where no delay voltage is used.

With a rectifier constructed in accordance with the present invention, however, a delay voltage may be used with its attending advantages explained above, and at the same time the rectified voltage available for AVC in the case of strong signals is just as great as if the delay voltage were not used.

Hence, it can be stated to be one of the main objects of the present invention to provide a rectifier circuit for alternating currents so arranged that the rectified, or direct current voltage, output is zero for values of alternating voltage up to a predetermined amount, but substantially equal to the peak value of alternating voltages appreciably in excess of the aforementioned amount.

A secondary object of the invention is to provide a radio receiver having automatic volume control effected by a bias voltage obtained from a rectifier of the type last described; this rectifier being energized by an alternating voltage derived from a signalling wave being received.

The novel features which we believe to be characteristic of our invention are set forth in par- 5 ticularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically several circuit organizations whereby our invention may be carried into effect.

In the drawing:

Fig. 1 shows a simplified circuit embodying the invention,

Fig. 2 shows the rectified voltage of the circuit of Fig. 1 plotted against the peak value of alternating voltage applied to the rectifier input,

Fig. 3 shows a portion of a radio set embodying 20 the present invention in a form suitable for commercial production.

Considering now the circuit in Fig, 1, it is known that no plate current will flow in triode T unless the plate'potential of plate P is greater than the instantaneous negative potential of the grid G multiplied by the amplification constant of the tube. Thus, unless the peak value of alternating voltage across tuned circuit S is greater than the product of the amplification factor and the difference between the bias, or delay, voltage E0 and the peak value of the alternating voltage impressed on the grid, no plate current will flow. If, however, the plate potential susbtantially exceeds the aforementioned value, plate current will fiow and, after several successive waves, will charge the condenser C to a potential determined by the strength of the waves and the value of resistance R. If the voltage across circuit S is now increased to the point where the grid bias disappears entirely, or the grid becomes positive at least at the peak of each wave, then the plate current flowing at the peak of the waves will charge the condenser C to a potential equal to the peak value of the voltage across circuit-3. It should be noted that there is a negligible loss of voltage by reason of a slight continuous discharge through resistance R, the latter is supposed to be so high that the time constant of RC is large compared to the wave frequency. 50

The operation of this rectifier can be divided into three ranges, In the first range of input voltage there is no direct current output voltage at all. In the third range the output voltage is substantially equal to the peak value of the input [a voltage, and independent of the delay bias Be. In the second, or transition, range the rectified voltage increases more rapidly than in linear relation to the increasing input voltage. The width of the second, or transition, range may be controlled by alteration of the position of the tap l determining the amount of alternating voltage impressed upon the grid G. Fig. 2 shows how the rectified voltage remains at zero for the first range of alternating input voltage, then increases rapidly over the second range, and finally converges upon the line of equality (dotted line) between rectified voltage and peak applied voltage.

If the time constant RC is small compared to the period of modulation of a signal the rectified voltage will swing up and down the curve of Fig. 2 as the signal is modulated, and its average value will obviously be lessened by modulation if the modulation is sufficiently deep to bring the rectified voltage down into the transition range. If the transition range is not reached until the signal strength is greatly reduced this effect will not be appreciable. If, however, a large delay bias is used so that the third range begins only at a relatively high signal level, then the average AVC voltage as averaged out by the low pass AVC filter will decrease in proportion to the depth of modulation, 1. e., will be less, the stronger the average audio output of the system. Thus in the case of music, loud passages will be executed in a manner tending to restore the full dynamic range which is ordinarily somewhat compressed at the transmitter. In Fig. 1 the plate end of resistor R is to be understood as being adapted for connection to the AVG lead. The signal source can be the usual pre-detector stages employed in a broadcast receiver. Fig. 2, in the case of an AVC receiver, depicts the relation between AVG volts and Signal strength in peak volts applied to the AVG rectifier.

In Fig. 3 there is shown a receiver circuit embodying the invention. The AVC rectifier 2 has a tuned input circuit 3; the latter may be adjustably tuned over a wide frequency range, or the circuit 3 may be fixedly resonated to a desired intermediate frequency Where the receiver is of the superheterodyne type. The controlled stages may include the usual transmission tubes which precede the detector. The numeral 4 denotes a radio frequency amplifier having a tuned input circuit. It will be understood that the plate circuit of tube 4 can be coupled to circuit 3 by any network. The operation of the AVG rectifier 2 in Fig. 3 is the same as explained in connection with Fig. 1.

For commercial reasons it is objectionable to employ a delay battery for the grid 5 of rectifier 2; a negative potential is, therefore, taken from a point on the power supply bleeder P and applied to the grid of the AVG rectifier, while the signal input to the grid is applied through a condenser 8. In this case the grid 5 itself tends to assume such a bias, for all signals greater than the delay voltage, that the instantaneous potential of the grid becomes only slightly positive at each wave peak regardless of the amplitude of the wave. The grid 5 is connected to negative point i on bleeder P through the path including resistor 8, lead 9 and tap Ill. This, however, does not prevent the plate circuit from functioning as before described, since any positive potential on the grid at the wave peak permits the plate II to draw electrons to itself whenever its potential is more positive than that of the grounded cathode. The average AVC potential developed by the AVC rectifier across resistor I2 is obtained by connecting the plate end resistor l2, through a low-pass filter III to the return circuit of the grid of amplifier tube 4. The resistor 8 functions to prevent excessive damping of the tuned circuit 3 by the drawing of large grid current during strong signals, and may be supplemented by the connection of an R. F. choke in series therewith if desired.

The plate I l of rectifier 2 is connected by condenser [4 to the high potential side of input circuit 3. A separate detector tube I5 is employed to secure the audio voltage from the received carrier waves. The grid of tube I5 is connected to the high potential side of input circuit 3 through a leaky grid condenser I6; the cathode of tube I5 is grounded The audio output voltage of detector 15 is transmitted through one or more audio amplifiers IT. The bypass condenser and the radio frequency choke 2| are employed in the plate circuit of the detector I5 to improve its rectification and to prevent impressing radio frequency energy upon the following audio tube. A large value of plate voltage is recommended, together with a large value of plate resistance 22 to stabilize the plate current of the tube l5 and to permit large voltage swing to be developed thereacross.

While we have indicated and described several systems for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claims.

What is claimed is:

l. A rectifier network comprising a tube pro vided with a cathode, plate and solely a single control grid, an alternating current input circuit connected between the cathode and plate, a load impedance connected between the cathode and plate, said cathode and plate being at the same direct current potential in the absence of current flow therebetween, means connecting said grid to said input ci cuit, means establishing the grid at a negative biasing potential with respect to the cathode, said biasing voltage determining a minimum value of alternating input peak voltage to said rectifier which will cause current to flow through said rectifier between the cathode and plate and develop direct current voltage across said impedance, the said direct current voltage of said rectifier being substantially equal to the said input peak voltage whenever said input voltage is sufiicient to make the grid potential positive at the peak of the input wave.

2. In combination with a triode, a signal input circuit connected between the cathode and plate thereof, a load circuit connected to the cathode and plate, said cathode and plate being at the sai 9 direct current potential in the absence of current fiow therebetween, means impressing at least a portion of the signal input circuit voltage upon the triode grid, and means biasing the latter to a negative voltage sufficient to delay rectification between the cathode and plate.

ROY A. WEAGANT. WALTER VAN B. ROBERTS, 

